1. Field of the Invention
The present invention relates to a catalyst support and more particularly to a catalyst support formed from aluminum silicate which is used, when impregnated with phosphoric acid, as a catalyst for the hydration of olefins.
2. Description of the Prior Art:
It is known that olefins are converted to alcohols by water vapor at high pressure in the gas phase. Such methods have particular technical significance in the production of ethyl alcohol from ethylene and isopropyl alcohol from propylene. The synthesis of these alcohols is carried out in the presence of catalysts. Normally serving as catalyst is phosphoric acid, which is applied to carriers.
Known support materials are either pure silicic acid (e.g. silica gel or infusorial earth) or silicic acid with a more or less large content of alumina, such as calcinized diatomaceous earth, whose structure is held together by clay or other clay-like materials.
With carriers based on silicic acid the stability over longer periods of standing becomes problematic. Materials containing alumina are characterized in particular by better mechanical strength; however, they have the disadvantage that when the alumina content is too high, the aluminum oxide is depleted by the action of the phosphoric acid during the reaction.
A procedure was described in DE-PS No. 11 56 772 for producing a carrier containing alumina for phosphoric acid used as catalyst in olefin hydration in which preformed carrier bodies made of mineral alumina silicates are treated with mineral acid in such a way that the aluminum oxide content is reduced to between 1 and 5% by weight. This material is generally characterized by the necessary mechanical stability as well as by an adequately low residual aluminum oxide content which avoids depletion. On the other hand, when commercially available carrier bodies made of mineral raw materials were used for the production of catalyst supports for the hydration of olefins, it was observed that without previous selection of the raw material, strongly differing catalyst activities were observed.
Finally, carriers were also successfully developed based on large pore silica gel carriers for phosphoric acid with high hydration activity and adequate mechanical strength, e.g. according to DE-OS Nos. 26 25 705 and 27 19 055. However, there is the remaining disadvantage with these carriers based on amorphous silicic acid that when exposed to hydration reaction conditions for long periods, the amorphous silicic acid partially crystallizes into cristobalite and quartz, which is associated with a strong, indeed irreversible, reduction of specific surface and thereby catalytic activity, as well with as a reduction of mechanical strength.
An additional disadvantage of these previously used hydration catalysts based on phosphoric acid applied to silicate carriers is the slow reduction of activity caused by depletion of phosphoric acid. The more recently developed technique of continuously spraying in the depleted amounts of phosphoric acid, according to DE-OS No. 26 58 946, was able to largely eliminate this continuous loss of activity and to considerably increase the life span of the catalysts. This, however, places corresponding demands on the life span of the carrier so that the usefulness of such carriers is determined by crystallization which occurs under reaction conditions and irreversibly reduces catalytic activity and by the mechanical strength which also reduces with time.
According to patent application No. P 29 08 491.1, clay minerals can be used to make a carrier for a hydration catalyst with consistantly high catalytic activity, if care is taken to choose a raw material consisting to a high degree of montmorillonite. The result is that after forming, leaching and impregnating, the active surface on which olefin hydration can occur is large.
Patent application No. P 29 08 491.1 relates to a method for producing a catalyst of clay minerals for the hydration of olefins with 2-3 C-atoms to the corresponding alcohols from phosphoric acid and support materials as well as the thus-produced catalyst. A clay, containing essentially montmorillonite contaminated with no more than 3% accompanying minerals such as quartz, feldspar, and mica, and containing up to 0.5% K.sub.2 O, is treated in a first step with acid until it has an Al.sub.2 O.sub.3 content of 13-18% by weight. If necessary, precipitated alumina is added to bring the Al.sub.2 O.sub.3 content to 16-18% by weight. The resulting material has a surface of 200-400 m.sup.2 /g, preferably 240-300 m.sup.2 /g. This is then press formed when its total water content is 20-35%, and calcinated at 500.degree.-800.degree. C. Then the thus formed carrier material is treated in a second step with acid until the Al.sub.2 O.sub.3 content is 1-5% by weight, preferably 1-3% by weight, whereby a surface of 150-250 m.sup.2 /g, preferably 180-220 m.sup.2 /g is obtained. Finally, the thus obtained carrier is soaked according to known methods in phosphoric acid.
Montmorillonite can be replaced by another mineral of the montmorillonite group containing no potassium, but having the montmorillonite crystalline lattice.
It is also possible to use an acid pre-treated fuller's earth made from a clay with a high montmorillonite content instead of non-acid-treated montmorillonite clay. This eliminates the first acid treatment. This fuller's earth should contain less than 0.1% of K.sub.2 O ; the weight ratio (Al.sub.2 O.sub.3 +Fe.sub.2 O.sub.3): SiO.sub.2 should be 1:3.5 to 1:4.5. If necessary, the Al.sub.2 O.sub.3 content of the fuller's earth can be brought to the necessary 16-18% by weight by adding precipitated alumina.
In comparison to catalysts and catalyst carriers made of preformed carrier bodies based on mineral clay silicates of different origins, the catalysts and carriers produced in this manner have increased activity, i.e. approximately 105-110 g of ethanol or ca. 300 g of isopropyl alcohol are produced per hour and liter of catalyst charge. However, this increased activity can only be maintained over a longer time if the phosphoric acid, depleted at a rate of ca. 0.07 g per hour and liter of catalyst charge with ethanol and 0.01 g per hour and liter of catalyst charge with isopropyl alcohol, is continuously replenished by adding an equal amount of acid.
Patent application No. P 29 29 919.2 deals with a further improvement of the method described above. To the three mentioned raw materials, i.e., either the montmorillonite clay with no more than 3% accompanying minerals such as quartz, feldspar, and mica, having a K.sub.2 O content below 0.5, or another mineral of the montmorillonite group containing no potassium, but having the montmorillonite crystalline lattice, or a previously acid-treated fuller's earth, made of clay with a high montmorillonite content, are added 5-15% by weight, based on the total dry substance, of one or more oxides of metals of Group VI of the periodic table before pressing and calcination at 500.degree.-800.degree. C.
With the method according to patent application No. P 29 29 919.2 the following specific improvements are attained:
(a) An increase of the spherical compression strength of the catalyst, PA1 (b) A reduction of phosphoric acid depletion under reaction conditions, PA1 (c) An increase of catalyst activity to ca. 130 g of ethanol per hour and liter of catalyst charge.